Add some missing return types to internal/final classes

Author: nicolas-grekas

Chunk/ErrorChunk.php

@@ -120,10 +120,7 @@ public function didThrow(bool $didThrow = null): bool
         return $this->didThrow;
     }
 
-    /**
-     * @return array
-     */
-    public function __sleep()
+    public function __sleep(): array
     {
         throw new \BadMethodCallException('Cannot serialize '.__CLASS__);
     }

Response/TraceableResponse.php

@@ -44,10 +44,7 @@ public function __construct(HttpClientInterface $client, ResponseInterface $resp
         $this->event = $event;
     }
 
-    /**
-     * @return array
-     */
-    public function __sleep()
+    public function __sleep(): array
     {
         throw new \BadMethodCallException('Cannot serialize '.__CLASS__);
     }